1. Field of the Invention
The invention relates to the fabrication of submersible structures useful for deep water installation. Such submersible structures may be incorporated into offshore oil drilling and production platforms.
2. Summary of the Art
It is known that one problem in the design of fixed offshore structures for water depths beyond 1,000 feet is in keeping the natural period of vibration within acceptable limits. This problem may be solved by increasing the width at the base to provide greater stability and structural stiffness; however, increasing the width is no simple matter.
The lower, or below water portion of a submersible structure, is normally fabricated lying on its side. The width of the base of such structure is limited by the working height of the cranes available for assembling the various components. The width of the base for one-piece fabrication is limited to approximately 300 feet for equipment available at the principal fabrication sites.
A submersible structure for a water depth of 1,025 feet has been fabricated in three pieces. The base of said structure had a width of approximately 400 feet, but it was fabricated in the vertical position to a height of only about 200 feet. This structure was installed on the sea bottom with two upper sections added to complete the structure.
A method of fabricating a tower structure for a maritime platform assembly in two pieces is disclosed in U.S. Pat. No. 4,080,916. The disclosed method includes fabricating two half-sections of the tower in a horizontal position and then rotating each half-section to the vertical and securing them to each other to provide a vertically standing tower. Since the tower is assembled on the ground, the height of the tower is limited by the available crane height, or separate rearing towers must be provided to assist in rotating the half-sections to the vertical. As shown, the cranes utilized in rotating the half-sections to the vertical are the track-mounted mobile cranes used in the prior art methods for fabricating submersible towers.
It is thus clear that current methods for the fabrication of broad-based submersible towers are limited by the height of mobile cranes or require additional structural towers to overcome crane height limitations.
Therefore it is a primary object of this invention to provide a method for fabricating broad-based submersible towers that is not limited by the height of the mobile cranes generally utilized in fabricating submersible structures.
It is another object of this invention to provide a method for fabricating submersible structures without the use of rearing towers.
Other objects and advantages of this invention will be apparent from the following specification.